This invention is a circuit for the high speed translation of the output signals of a color raster input scanner (RIS) into signals representing amounts of colored inks to allow the production of an accurate color circuit specifically comprising a set of look-up tables for the conversion of the red, green and blue inputs from the RIS to the cyan, magenta, yellow and black output of the ROS.
Color copiers may convert the scanned input signals into color copies, but the colors are only approximate over the total range of densities.
The accurate translation from red, green and blue to cyan, magenta, yellow and black is known mathematically, but has required extensive memory and computer power to accomplish. Cyan is computed as a constant times red minus a constant times green minus a constant times blue. Similarly, magenta is a function of green, red and blue, and yellow is a function of blue, red and green.
A complicating factor is that, for the best color rendition, a process known in the industry as "under color removal" is also required. Simply stated, the process is to first find which of the three output colors is using the least ink. For a numerical example, assume that the output scanner has computed an output comprising one and a half units of cyan ink, three units of magenta and three units of yellow. Then the least ink is one and one half units. The next step is to subtract a fraction of that amount of least ink from all three colors, and substitute an equal amount of black ink. In the numerical example, assume that the amount of color removed is seven tenths of the color over an original threshold of one half a unit. Then 0.7 units of ink will be removed from each color, and 0.7 units of black ink will be used. Therefore, the final amounts of ink will be 0.8 units of cyan, 2.3 units of magenta, 2.3 units of yellow and 0.7 units of black. It is only after this computation is accomplished that the amount of any color ink is known.
In a raster output scanner, this represents an extensive amount of computation for each dot printed, in that the three output colors and the amount of under color removal must all be determined before the amount of ink for the dot being printed can be determined. In a four color system, the computation time will slow down the printing rate.
What is required is a relatively economical circuit that will accomplish this entire conversion at standard printer operating speeds.